In a typical cellular communication system, also referred to as a wireless communication system, User Equipments (UEs) communicate via a Radio Access Network (RAN) to one or more core networks (CNs).
A user equipment may be a mobile terminal by which a subscriber can access services offered by an operator's core network.
The user equipments may be mobile stations or user equipment units such as mobile telephones, also known as “cellular” telephones, and laptops with wireless capability, and thus may be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with the radio access network.
Each cell in the cellular communication system covers a geographical area. A cell is served by Radio Base Station (RBS) equipment at a radio base station site. That is, the radio base station provides radio coverage in the cell and communicates over an air interface with user equipment units operating on radio frequencies within its range.
A radio base station is in some cellular communication systems also called “eNB”, “eNodeB”, “NodeB” or “B node”, and will in this document be referred to as a Base Station (BS).
The part of the cellular communication system that handles the communications with the user equipments over radio link is referred to the Radio Access Network.
Over the years, several different Radio Access Technologies (RATs) have evolved, such as GSM, WCDMA and LTE.
Depending on the RAT in question, the RAN comprises different radio network nodes, which may be referred to by different names.
In GSM, the radio base stations are referred to as base stations, and are controlled by so called Base Station Controllers (BSCs).
In WCDMA, the base stations are controlled by so called Radio Network Controllers (RNCs).
In LTE, the base stations are referred to as eNodeBs. There is no node in LTE corresponding to the BSC or RNC units of GSM and WCDMA. In LTE, the base stations, eNodeBs, themselves comprise further functionality and are interconnected to provide the required services in the RAN.
The different RATs may all exist in the same geographical area, and hence the user equipment may access the core network through different RATs depending on, for example, which RAT provides the best coverage at a specific moment in time or at a specific location.
To enable coexistence of the RATs, the different RATs have their own cells, or frequencies, that are used for communication with the user equipments.
Cell selection, meaning that the user equipment selects a cell to camp on, enables the user equipment to select another cell than the currently serving cell, for example if another cell in another RAT provides better quality of service or coverage.
At times when the user equipment is not actively using the services provided by the core network, it usually enters a so called “idle mode”.
In idle mode the user equipment continuously performs measurements on neighbour cells to enable cell selection using standardized idle mode cell reselection algorithms. As input to the cell reselection algorithm, the user equipment uses system information which is broadcast in each cell for all user equipments which are in idle mode and listening.
While in idle mode, the user equipment may indicate its presence to the network by temporarily leaving idle mode to perform so called area update procedures, either periodically or when the user equipment changes area. An area update may be referred to as a “Location Area” (LA) update, or “Tracking Area” update.
When the user equipment starts using the services of the core network, and for example gets involved in an activity such as a phone call, it leaves idle mode and enters into a so called “dedicated mode”.
When a user equipment is in dedicated mode, there are radio links, channels, set up between the user equipment and the RAN to enable the necessary data transfer to provide the requested service. Such channels may be referred to as for example a Traffic Channel (TCH) and a Standalone Dedicated Control Channel (SDCCH).
When the call ends, a so called “channel release” message is signaled from the RAN to the user equipment to release the data transfer channels, whereupon the user equipment re-enters idle mode.
When the user equipment is in dedicated mode, it does not listen to the broadcast system information, and does not perform cell selection.
When a call is released and the user equipment re-enters idle mode, it normally selects the serving cell it had when the call was released, and performs an area update if the area has changed during the call.
After the area update, the user equipment reads the broadcasted system information and, if information about other RATs is provided in the system information, the user equipment may select a cell in another RAT by using the measurements and broadcast system information as input to an idle mode cell reselection algorithm as mentioned above.
A move to another RAT may also be initiated at channel release by the RAN, by including a cell selection indicator in the channel release message for the user equipment, indicating which frequencies/cells the user equipment should select.
In order for the RAN to select a cell or frequency, to include in the channel release message, in the same RAT as would be selected by the user equipment idle mode cell reselection algorithm, the network may need to base the cell selection on user equipment measurements. Such measurements need hence be sent in measurement reports from the user equipment while it is in dedicated mode, to the controlling radio network node in the RAN. Based on the measurements, the RAN may then decide which cell provides the best radio connection for the user equipment, and direct it to a specific cell in a specific RAT.
The cellular communication system may also select RAT for the user equipment based on configuration.
A problem is that, during the actual cell reselection process and until the area registration is completed in the selected RAT, the user equipment cannot be reached from the cellular communication system. The time period during which the user equipment is unreachable due to this is referred to as the “outage time”.